Project 2 Summary Genetic studies from our group has shown that mutations in the genes encoding either of two type I procollagen chaperones, FKBP10 (which encodes the FKBP65 protein) or SERPINH1 (which encodes HSP47), produce severe, recessively inherited forms of OI. Furthermore, we have determined that the two proteins form a complex acting on type I procollagen trimers in the endoplasmic reticulum (ER) and whose function is essential for normal type I procollagen biogenesis. Abrogation of the complex function leads to an altered cellular phenotype with dilated ER, aggregates of intracellular type I procollagen, sequestering of chaperone complex components and abnormal PLOD2-dependent cross-linking. Through the use of mutant FKBP65 and HSP47 cell lines, we have established that PLOD2 is also a member of this newly identified chaperone complex and identifies chaperone dysfunction as a new mechanism of disease in OI. These studies will primarily use newly generated mouse models, complemented by studies in human OI tissues to establish a currently unappreciated mechanistic paradigm for type I procollagen synthesis and a detailed understanding of how OI results from defects in this process. This is a paradigm shift in our understanding of type I procollagen synthesis from the viewpoint of how LH2 modifies or has access to type I procollagen and provides insight into how OI with contractures, also known as Bruck syndrome, can result from mutations in either FKBP10 or PLOD2. The proposed experiments are significant because they have the potential to have an extensive impact on our understanding of the role of telopeptide cross-linking plays in the generation of a functional extracellular matrix and will be essential to understanding and tailoring therapeutics in the context of altered bone matrix on downstream function of bone and associated tissues.